writev.go

Documentation: internal/poll

     1  // Copyright 2016 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // +build darwin dragonfly freebsd linux netbsd openbsd
     6  
     7  package poll
     8  
     9  import (
    10  	"io"
    11  	"syscall"
    12  	"unsafe"
    13  )
    14  
    15  // Writev wraps the writev system call.
    16  func (fd *FD) Writev(v *[][]byte) (int64, error) {
    17  	if err := fd.writeLock(); err != nil {
    18  		return 0, err
    19  	}
    20  	defer fd.writeUnlock()
    21  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
    22  		return 0, err
    23  	}
    24  
    25  	var iovecs []syscall.Iovec
    26  	if fd.iovecs != nil {
    27  		iovecs = *fd.iovecs
    28  	}
    29  	// TODO: read from sysconf(_SC_IOV_MAX)? The Linux default is
    30  	// 1024 and this seems conservative enough for now. Darwin's
    31  	// UIO_MAXIOV also seems to be 1024.
    32  	maxVec := 1024
    33  
    34  	var n int64
    35  	var err error
    36  	for len(*v) > 0 {
    37  		iovecs = iovecs[:0]
    38  		for _, chunk := range *v {
    39  			if len(chunk) == 0 {
    40  				continue
    41  			}
    42  			iovecs = append(iovecs, syscall.Iovec{Base: &chunk[0]})
    43  			if fd.IsStream && len(chunk) > 1<<30 {
    44  				iovecs[len(iovecs)-1].SetLen(1 << 30)
    45  				break // continue chunk on next writev
    46  			}
    47  			iovecs[len(iovecs)-1].SetLen(len(chunk))
    48  			if len(iovecs) == maxVec {
    49  				break
    50  			}
    51  		}
    52  		if len(iovecs) == 0 {
    53  			break
    54  		}
    55  		fd.iovecs = &iovecs // cache
    56  
    57  		wrote, _, e0 := syscall.Syscall(syscall.SYS_WRITEV,
    58  			uintptr(fd.Sysfd),
    59  			uintptr(unsafe.Pointer(&iovecs[0])),
    60  			uintptr(len(iovecs)))
    61  		if wrote == ^uintptr(0) {
    62  			wrote = 0
    63  		}
    64  		TestHookDidWritev(int(wrote))
    65  		n += int64(wrote)
    66  		consume(v, int64(wrote))
    67  		if e0 == syscall.EAGAIN {
    68  			if err = fd.pd.waitWrite(fd.isFile); err == nil {
    69  				continue
    70  			}
    71  		} else if e0 != 0 {
    72  			err = syscall.Errno(e0)
    73  		}
    74  		if err != nil {
    75  			break
    76  		}
    77  		if n == 0 {
    78  			err = io.ErrUnexpectedEOF
    79  			break
    80  		}
    81  	}
    82  	return n, err
    83  }
    84
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